New drill may reveal Tahoe quake history

TAHOE Antarctic drill to probe ancient landslide under lake

Published 9:27 pm, Sunday, September 2, 2012

A submersible remotely operates vehicle floats on the surface of Lake Tahoe during a seismic study of the bottom of the lake.

A submersible remotely operates vehicle floats on the surface of Lake Tahoe during a seismic study of the bottom of the lake.

Photo: Paul Chinn, The Chronicle

Image 2 of 8

Mike Osment and Joseph Loanzon prepare equipment to recover samples of ancient life in the sediments at Lake Tahoe. Layers of the sediments could reveal the rate of quakes in the past.

Mike Osment and Joseph Loanzon prepare equipment to recover samples of ancient life in the sediments at Lake Tahoe. Layers of the sediments could reveal the rate of quakes in the past.

Photo: Paul Chinn, The Chronicle

Image 3 of 8

Image 4 of 8

Researchers move a remotely operated submersible into position for a seismic study at the bottom of Lake Tahoe.

Researchers move a remotely operated submersible into position for a seismic study at the bottom of Lake Tahoe.

Photo: Paul Chinn, The Chronicle

Image 5 of 8

Researchers aboard a barge anchored two-and-a-half miles from shore conduct a seismic study at the bottom of Lake Tahoe near Tahoe City, Calif. on Wednesday, Aug. 29, 2012.

Researchers aboard a barge anchored two-and-a-half miles from shore conduct a seismic study at the bottom of Lake Tahoe near Tahoe City, Calif. on Wednesday, Aug. 29, 2012.

Photo: Paul Chinn, The Chronicle

Image 6 of 8

Gordon Seitz, from the California Geological Survey, examines a rock estimated to be 12,000 years old, in Tahoe City, Calif. on Wednesday, Aug. 29, 2012. Seitz says the rock, which was recovered from the floor of Lake Tahoe during a seismic study, was probably deposited there by a glacier. less

Gordon Seitz, from the California Geological Survey, examines a rock estimated to be 12,000 years old, in Tahoe City, Calif. on Wednesday, Aug. 29, 2012. Seitz says the rock, which was recovered from the floor ... more

Photo: Paul Chinn, The Chronicle

Image 7 of 8

Tony Lawson operates a submersible ROV for a seismic study at the bottom of Lake Tahoe near Tahoe City, Calif. on Wednesday, Aug. 29, 2012.

Tony Lawson operates a submersible ROV for a seismic study at the bottom of Lake Tahoe near Tahoe City, Calif. on Wednesday, Aug. 29, 2012.

Photo: Paul Chinn, The Chronicle

Image 8 of 8

New drill may reveal Tahoe quake history

1 / 8

Back to Gallery

A web of dangerous seismic faults deep beneath the placid waters of Lake Tahoe has triggered earthquakes in the past and will do so again.

Over the past 60 years, small quakes have rattled communities around the lake while larger ones have been felt in towns like Truckee, farther inland from the shore. But scientists know little about when and how often big quakes struck the region along one of the lake's three major faults.

In their quest for fresh clues, they have begun a search for answers in the rocky rubble of an ancient giant landslide beneath Tahoe's western shore, using a powerful new tool designed to explore a far distant lake hidden under a half-mile of Antarctic ice.

For the first time, California geologists have hammered a drill into thick sediments atop the landslide's rocks to recover samples of ancient life.

The dated layers of sediments could reveal the recurrence rate of past quakes along what scientists call the West Tahoe fault, which runs for more than 20 miles beneath the lake, roughly between Emerald Bay and Tahoe City, and continues north and south on land.

Knowing when and how often that fault ruptured could lead to upgraded seismic hazard maps for the entire Tahoe region, said Gordon Seitz, an engineering geologist with the California Geological Survey who has long studied seismicity at the lake.

Seitz and his science team completed their first exploratory drilling effort Thursday, and will return in a month to probe the bottom again for more samples.

Ancient avalanche

The enormous landslide at McKinney Bay occurred at least 50,000 years ago, Seitz said, and sent an avalanche of rocks that buried more than 5 miles of the lake bottom, with some boulders ending up on what is now the Nevada shore, Seitz said.

Successive layers of sediment samples, holding the remains of long-buried living organisms, will be clearly dated by specialists at the Lawrence Livermore National Laboratory, he said. Rock layers that have been deformed by quakes can be read like a book, he said.

There's also a story of luck and teamwork behind the California geologist's venture, said Ross Powell, a geologist at Northern Illinois University and a leader of a far different $10 million exploration venture in Antarctica called Wissard, the Whillans Ice Stream Subglacial Research Drilling project.

Powell has been testing his group's underwater instruments at Lake Tahoe this week for his Antarctic project. His venture calls for a unique robot submarine, a cigar-shaped vehicle only 22 inches wide and designed to be lowered through the West Antarctic Ice Sheet, to explore a lake beneath the ice called Lake Whillans. The lake, which has never been seen before, may hold samples of unknown life forms millions of years old.

Tahoe water like Antarctica

The sub, named SIR for Sub Ice Robot, was built by a specialized engineering firm in Alameda called Doer Marine. The firm's president, Liz Taylor, knows members of the California Seismic Safety Commission, Powell said, and she suggested that the new robot and its ice-penetrating drill could also bring up samples from Tahoe for geologists.

"Tahoe's fresh water was glaciated and it was pretty much like Antarctica during the last ice age," Powell said. "So it's an ideal place to test our robot's instruments for exploring the fresh-water lake there."

The drill, called a percussion corer, does not screw into the ground, but carries a 2,000-pound weight above its drill bit, which "basically hammers the drill down into rocks or ice again and again," Powell said.

In the Tahoe trials it could penetrate the McKinney Bay landslide's surface for more than 16 feet, he said.

The three major seismic faults at Tahoe are called Normal faults, Seitz said. Unlike the San Andreas Fault, where two massive blocks of the Earth's crust slide past each other, one block of crust abruptly drops down below the other.

"And a big earthquake beneath the lake could cause a towering rush of water like a 30-foot tsunami, followed by a succession of more waves, one after the other, with each one called a seiche," Seitz explained.

"On land when you feel a strong quake, the rule is 'duck, cover and hold,' " he said. "Up along the Tahoe shore, when you feel a strong quake, the rule is 'duck, cover, hold on, and then move quickly to higher ground.' "

Latest from the SFGATE homepage:

Click below for the top news from around the Bay Area and beyond. Sign up for our newsletters to be the first to learn about breaking news and more. Go to 'Sign In' and 'Manage Profile' at the top of the page.